How Scientists Know Alien Planet Kepler-10b is a Small, Rocky World

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SEATTLE — Astronomers announced the discovery today (Jan. 10) of
the first "unquestionably rocky" alien world — a planet called
Kepler-10b.

Scientists have announced other rocky exoplanets in the past, but
Kepler-10b earns its title, researchers said, because they're
sure of its composition. And that's because they know its host
star so well. In fact, the Kepler-10 star is one of the most
well-characterized planet-hosting stars in the universe.

Once researchers first picked up
Kepler-10b's signal, back in 2009, they trained the Kepler
Space Telescope on its parent star, located about 560 light-years
from Earth. Scientists took extremely detailed measurements of
the star's brightness oscillations, using this information to
probe its interior, in much the same way that geologists use
earthquakes to study Earth's interior.

This method, called
asteroseismology, allows researchers to learn a great deal
about a star's structure, including its size. Once the scientists
had this information in hand, they used it to deduce how big, and
how dense, Kepler-10b was.

Researchers now know the properties of the parent star within 2
to 6 percent, Natalie Batalha, leader of the study announcing the
new planet's discovery, told reporters here today at the winter
meeting of the American Astronomical Society.

Batalha and her colleagues made their discovery using NASA's
Kepler Space Telescope, whose primary mission is to hunt down
potentially habitable Earth-like planets in our galaxy. Kepler
finds alien planets that pass in front of their host stars,
picking up on the tiny brightness dips that occur when such
"transits" take place.

The magnitude of those brightness dips — in Kepler-10b's case, it
was a drop of 0.015 percent, Batalha said — tells researchers how
big the planet is, relative to its parent star.

Measurements made by the Keck Telescope in Hawaii determined how
much the newly discovered planet tugged on its star, making it
wobble. These observations yielded the planet's mass — again,
relative to the parent star.

If a planet's mass and volume are known, it's a simple step to
calculate its density, which tells researchers the likely
composition (rocky, gaseous, icy, etc.). But to do this for the
hellishly hot Kepler-10b, Batalha and her colleagues needed to
convert relative mass and density into absolutes.

And that's where Kepler's asteroseismology observations came in.
Since the team got such a detailed and accurate look at the star,
they feel confident that they're getting Kepler-10b's density
correct. The researchers calculated the density of Kepler-10b at
8.8 grams per cubic centimeter, which places it squarely in the
"rocky" category, just like Earth.

"Kepler-10b is the smallest exoplanet discovered to date, and the
first unquestionably rocky planet orbiting a star outside our
solar system," Batalha said. "The composition could even be
consistent with an Earth composition."

What about Corot-7b?

Kepler-10b isn't the first exoplanet scientists have claimed is
rocky. In 2009, for example, a different research team announced
that the smallish, scorched alien world Corot-7b
is rocky as well.

When asked about Corot-7b, Batalha didn't claim that it wasn’t
rocky. But, she did say that there's enough uncertainty in the
measurements to leave some doubt — hence Kepler-10b's status as
the first "unquestionably rocky"
alien planet ever found.

"Corot-7b was indeed a very important discovery," Batalha said.
It could turn out to be rocky, she added, but it's difficult to
say for sure at the moment. "It's a tough one."